The way that the DB tables have been split up across the codebase -
which I still believe is the right call - has led to us not being able
to `JOIN` across tables, which leads to us missing out on functionality
where we want to `JOIN`, or we `JOIN` by retrieving the rows into Go and
then passing them into another query, which isn't ideal.

To avoid this, we can add a `table-joiner` utility that allows us to
`go generate` an SQL file to join the different schemas needed for a
`JOIN`.